The present invention relates to borehole logging, and in particular to the measurement and identification of the earth formations penetrated by such a borehole, using acoustical investigation methods and apparatus.
Velocities of acoustic waves propagating in the formation around a well bore reflect a number of properties of the formation. Measurements of compressional and shear wave velocities aid in determination of such formation properties as porosity, lithology, formation fluid, and formation mechanical properties. These velocities also find use in identifying fractured zones around the borehole, and in enhancing seismological surveys.
Identifying the compressional wave and measuring its velocity is generally not difficult. It is the fastest propagating wave in the formtion, is nondispersive, and is the first to reach an array of borehole receivers when a short burst of energy from a nearby transmitter propagates through the formation. By measuring the arrival times of these waves at the receivers, the wave velocity in the vicinity of the array can be inferred.
Measuring shear velocity is considerably more difficult. Because it propagates more slowly through the formation, the shear wave arrives later in time. Therefore, its arrival is typically obscured by compressional energy, and velocity determination directly from arrival time then becomes impossible.
The prior art discloses several methods for determining acoustic wave parameters in well logging operations. One such method, first-motion detection, is particularly applicable to measuring the transit time of formation compressional waves. Another technique, multiple fold correlation, has been applied to segments of compressional waveforms detected at various spaced positions to refine the results of the first motion detection. It has also been used for determining other acoustic parameters such as the shear wave velocity (e.g., by selecting starting places deeper within the waveform). Additional techniques include pattern comparison, measurement of correspondence between waveform segments as a function of waveform locations, and, particularly in the seismological field, frequency and phase analysis to detect speed and direction of propagation.
While such methods furnish good information about the characteristics of the earth formations being measured, they generally require the use of more complex equipment. For example, multiple fold correlation typically requires more than two receivers to obtain sufficient accuracy. Many such techniques also do not work well in broad ranges of lithologies, or are difficult to interpret. A need therefore remains for a more accurate, more versatile, more reliable, and less complicated method and apparatus for compressional and shear acoustic velocity borehole logging.